Aggressive black beagle finally upset collies (9)
I believe the answer is:
bellicose
'aggressive' is the definition.
('bellicose' can be similar in meaning to 'aggressive')
'black beagle finally upset collies' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'finally' suggests the final letters.
'upset' is an anagram indicator.
The last letter of 'beagle' is 'e'.
'collies' is an anagram of 'llicose'.
'b'+'e'+'llicose'='BELLICOSE'
(Other definitions for bellicose that I've seen before include "wanting fight" , "Aggressive and willing to fight" , "Aggressive, looking for a fight" , "Warlike" , "Not like lamb" .)